Ten Hag said: "It was after a very good performance against Chelsea in the week (Man Utd's 2-1 victory at Old Trafford).
"We were mentally not ready for that game (Bournemouth's 3-0 win). They battled us and we lost the battles.
"Tomorrow will not be different, that is the way they want to play. They want to fight with you.
"So you go into that fight, have the conviction, the belief and you need to support each other to win battles, to outplay them, outrun them and defending to match the runs."
On the fitness of Scott McTominay and Marcus Rashford, he added: "Scott? I dont think so. We have training, so we have a day and he will push. He is so keen to be part of it tomorrow, but I think it is very questioning.
"Rashy trained yesterday and (we will) see how he recovered from it. We will have training again today and then we will make a decision on it."